1. Field of the Invention
This invention is directed to bitumen compositions which are prepared from bitumen, a tri-block copolymer of styrene and a conjugated-diene, and defined amounts of sulfur. The bitumen compositions described herein are useful in industrial applications, such as in hot mix asphalts useful in preparing aggregates for road paving.
2. State of the Art
The use of bitumen (asphalt) compositions in preparing aggregate compositions (bitumen +rock) useful as road paving material is complicated by at least three factors, each of which imposes a serious impediment to providing an acceptable product. Firstly, the bitumen compositions must meet certain performance criteria or specifications in order to be considered useful for road paving. For example, to ensure acceptable performance, state and federal agencies issue specifications for various bitumen applications including specifications for use as road pavement. Current Federal Highway Administration specifications designate a bitumen (asphalt) product AC-20R as meeting defined parameters relating to properties such as viscosity, toughness, tenacity and ductility. Each of these parameters define a critical feature of the bitumen composition and compositions failing to meet one or more of these parameters will render that composition unacceptable for use as road pavement material.
Conventional bitumen compositions frequently cannot meet all of the requirements of a particular specification simultaneously and, if these specifications are not met, damage to the resulting road can occur, including permanent deformation, thermally-induced cracking and flexural fatigue. This damage greatly reduces the effective life of paved roads.
In this regard, it has long been recognized that the properties of conventional bitumen compositions can be modified by the addition of other substances, such as polymers. A wide variety of polymers have been used as additives in bitumen compositions. For example, copolymers derived from styrene and conjugated dienes, such as butadiene or isoprene, are particularly useful, since these copolymers have good solubility in bitumen compositions and the resulting modified-bitumen compositions have good rheological properties.
It is also known that the stability of polymer-bitumen compositions can be increased by the addition of sulfur, frequently in the form of elemental sulfur. It is believed that the sulfur chemically couples the polymer and the bitumen through sulfide and/or poly-sulfide bonds. The addition of extraneous sulfur is required to produce the improved stability, even though bitumens naturally contain varying amounts of native sulfur.
Thus, U.S. Pat. No. 4,145,322, issued Mar. 20, 1979 to Maldonado et al., discloses a process for preparing a bitumen-polymer composition consisting of mixing a bitumen, at 266.degree.-446.degree. F. (130.degree.-230.degree. C.), with 2 to 20% by weight of a block copolymer, having an average molecular weight between 30,000 and 300,000, with the theoretical formula S.sub.x -B.sub.y, in which S corresponds to styrene structure groups and B corresponds to conjugated diene structure groups, and x and y are integers. The resulting mixture is stirred for at least two hours, and then 0.1 to 3% by weight of sulfur relative to the bitumen is added and the mixture agitated for at least 20 minutes. The preferred quantity of added sulfur cited in this patent is 0.1 to 1.5% by weight with respect to the bitumen. The resulting bitumen-polymer composition is used for road-coating, industrial coating, or other industrial applications.
Similarly, U.S. Pat. No. 4,130,516, issued Dec. 19, 1978 to Gagle et al., discloses an asphalt (bitumen)polymer composition obtained by hot-blending asphalt with 3 to 7% by weight of elemental sulfur and 0.5 to 1.5% by weight of a natural or synthetic rubber, preferably a linear, random butadiene/styrene copolymer. U.S. Pat. No. 3,803,066, issued Apr. 9, 1974 to Petrossi, also discloses a process for preparing a rubber-modified bitumen by blending rubber, either natural or synthetic, such as styrene/butadiene rubber, with bitumen at 293.degree.-365.degree. F. (145.degree.-185.degree. C.), in an amount up to 10% by weight based on the bitumen, then adjusting the temperature to 257.degree.-320.degree. F. (125.degree.-160.degree. C.), and intimately blending into the mix an amount to sulfur such that the weight ratio of sulfur to rubber is between 0.3 and 0.9. A catalytic quantity of a free-radical vulcanization-accelerator is then added to effect vulcanization. This patent recites the critical nature of the sulfur to rubber ratio, and teaches that weight ratios of sulfur to rubber of less than 0.3 gives modified bitumen of inferior quality.
Although polymer-modified bitumen compositions are known, these previously described compositions are not necessarily useful for road paving applications. For example, mixing North West paving asphalt having an initial viscosity of 682 poise at 140.degree. F. (60.degree. C.) with 3.6 weight percent Kraton.RTM.-4141, a commercially available styrene-butadiene tri-block copolymer which contains 29 weight percent plasticizer oil, and 0.25% sulfur gives a modified-asphalt composition with a viscosity of 15,000 Poise at 140.degree. F. (60.degree. C.). This viscosity, however, greatly exceeds the acceptable viscosity range set by the widely-used AC-20R specification for paving asphalt. This specification, issued by the Federal Highway Administration, requires bitumen compositions to have a viscosity in the range of 1600-2400 Poise at 140.degree. F. (60.degree. C.). Thus, the modified bitumen compositions produced by the procedures of U.S. Pat. No. 4,145,322 using Kraton.RTM.-4141 would be unacceptable for use in road paving under the AC-20R specification.
The second factor which complicates the use of bitumen compositions concerns the viscosity stability of such compositions under storage conditions. In this regard, bitumen compositions are frequently stored for up to 7 days or more before being used and, in some cases, the viscosity of the composition can increase so much that the bitumen composition is unusable for its intended purpose. On the other hand, a storage stable bitumen composition would provide for only minimal viscosity increases and, accordingly, after storage it can still be employed for its intended purpose.
The third factor which complicates the use of bitumen compositions concerns the use of volatile solvents in such compositions. Specifically, while such solvents have been heretofore proposed as a means to fluidize bitumen-polymer compositions containing relatively small amounts of sulfur which compositions are designed as coatings (Maldonado et al., U.S. Pat. No. 4,242,246), environmental concerns restrict the use of volatile solvents in such compositions. Moreover, the use of large amounts of volatile solvents in bitumen compositions may lower the viscosity of the resulting composition so that it no longer meets viscosity specifications designated for road paving applications.
In view of the above, bitumen compositions which simultaneously meet the performance criteria required for road paving and which are substantially free of volatile solvent would be advantageous. Additionally, viscosity stable bitumen compositions would be particularly advantageous.